Looks like Arrow is going to get the al Ghul sister it always really wanted to have in the first place: Talia, sometimes Batman flame/international assassin, will be coming to the current season of Arrow. Talia al Ghul art from Gotham City Sirens #16 by Guillem March. Lexa Doig image provided by DC Comics.
Continuum's Lexa Doig will play the recurring role, and will first appear in the 10th episode of the current season, titled "Who are You?" CW describes Talia just as she is in the comics: Half-sister of Nyssa (already a recurring character on Arrow), as well as the daughter of Ra's, and a "worldly and cultured" warrior who "doesn't pick sides but rather creates her own".
In the comics, Talia has long been a prominent supporting character for Batman, alternating between anti-hero and villain while also being romantically entangled with Bruce Wayne (she is also the mother of his son Damian, the fifth Robin). Presumably at least some element of that will come to play in her presence in Arrow, determining whether she is friend or foe to Ollie and his team - rather than her and Ollie getting romantically involved, as much as Arrow would like to continue trying to be "Batman but with a bow and arrow" on occasion.